The disclosed invention is directed generally to holographic center high mounted stoplight systems for vehicles, and more particularly to a holographic center high mounted stoplight system having focused playback illumination having reduced light leakage.
Present federal regulations require center high mounted stoplights (CHMSLs) in addition to the standard stoplights mounted in the rear portion of an automobile. The CHMSLs are intended to maximize the visibility of the automobile braking indicators to drivers following the braking vehicle, and are commonly mounted on the rear window of an automobile.
CHMSLs have commonly been implemented as a standard lenticular lens and an illuminating incandescent bulb enclosed in a housing that is typically secured adjacent the top or bottom of an automobile rear window. However, the bulky housing partially obscures the rearward vision of the driver, and moreover imposes limitations on the design of the automobile.
CHMSLs have been also integrated into automobile body parts such as rear decks, spoilers, roofs, which to some degree substantially reduce or remove the rearward vision problem. However, such stoplights are complex and may impose limitations on the design of the automobile.
Holographic CHMSL systems have also been developed in order to efficiently meet the stoplight regulations. A holographic CHMSL system typically includes a stoplight hologram secured to the inside surface of the rear window of a vehicle, a playback light source activated by the braking system of the vehicle, and possibly a red filter located in the light path between the playback light source and the hologram. In order to minimize the power of the playback light source while producing diffracted stoplight illumination of the required luminous intensity, the light is preferably placed as close as practicable to the hologram, but out of driver's rearview mirror field of view. For example, the playback light source can be near the rear header that comprises the roof support structure in the roof of a vehicle which laterally traverses the vehicle.
A consideration with a roof lit CHMSL, however, is that the light source can be close to the hologram and out of the driver's rearview mirror field of view only for rear windows that are not too steep (i.e., not too close to horizontal). If the vehicle rear window is too steep, it becomes more difficult to achieve a sufficiently bright hologram while maintaining the playback source out of the driver's rearview mirror field of view. Lowering the playback illumination source to accommodate the steeper angle of the hologram would place the light source in the driver's rearview mirror field of view, and may impinge on the head motion arc of the rear center occupant, which is traced by a point on the top of the 95th percentile rear center occupant's head as he bends forward and backward in the vehicle. Impingement of the rear center occupant head motion arc introduces the potential problem of partial blockage of the playback light by the head of the rear center occupant, as well as the potential problem of head contact with the light source, which can become very hot.
Positioning the light source further forward of the hologram would alleviate the head bumping problem and possibly remove the light source from the driver's rearview mirror field of view, but the problem of head blockage would still remain. Moreover, the power of the light source would have to be increased to compensate for the increased distance to the hologram as well as to compensate for the increased stray light that is outside of the region subtended by the hologram.
One technique that attempts to address the problem of steep rear windows involves the use of multiple light guides for guiding the playback illumination to the hologram, which allows for convenient location of the light source. A consideration with light guides, however, is the cost of the light guides.